One of the most beautiful settlements of Sifnos with houses arranged on a hill in the form of a fortified settlement. Each house is next to the other, with their walls simultaneously forming the wall of the castle for protection from pirate attacks. In the castle, there are narrow alleys, recently whitewashed, small balconies and arches, and access is possible through the six entrance sheds. Kastro, the old capital of Sifnos, was inhabited as early as 3,000 BC and its ruins are scattered around the settlement. At the top is the Ancient Acropolis, with a wall made of Sifnian marble, as well as ancient columns. According to the reports, the settlement that was developed here during antiquity had a Prytaneion theatre of Dionysus and majestic homes. Today, the settlement maintains its scenic beauty, and despite its touristic development, it is a gem. The view of the settlement, as well as the view from the settlement, is something that becomes engraved in the memory and creates a sweet desire to return there. It is two kilometers from Apollonia.